1. Field of the Invention
This invention relates to ceramic abrasive grains of the Al.sub.2 O.sub.3 -Er.sub.2 O.sub.3 system used for grinding wheels, coated abrasives, lapping compositions and so forth and a method of producing the same and further an abrasive product made of the same.
2. Prior Art
Conventionally there has been provided a method for producing a sintered aluminous abrasive grain wherein alumina containing materials such as bayer alumina powder, bauxite powder and so forth are formed into particles with a binder and then sintered at a high temperature of 1600.degree. C. or higher. Another method has been also provided wherein alumina monohydrate used as a raw material is treated by a sol-gel process and sintered at a low temperature of 1500.degree. C. or lower. The former method produces abrasive grains including alpha alumina of a large size such as 3-10 microns and consequently the application thereof is limited to heavy duty grinding. The latter method produces abrasive grains having a crystal size of alpha alumina below 2 microns. There have been provided such methods of producing ceramic abrasive grains based on high density alumina by a sol-gel process as follows.
Japanese Patent Publication No. 1-54300 discloses a sintered alumina abrasive grain produced from a substantially calcium ion- and alkali metal ion-free alumina monohydrate by a sol-gel process. In this technology, it is essential that at least one modifying component is added into a colloidal dispersion of alumina in order to obtain a desirable grinding effect. The modifying component is added in a form of salt. Proposed for the modifying component is at least 10 vol % of zirconia and/or hafnia and at least 1 vol % of spinel derived from alumina and at least one oxide of a metal selected from cobalt, nickel,zinc, or magnesium.
Japanese Patent Publication No. 4-4103, which corresponds to U.S. patent application Ser. No. 572,106 which relates to U.S. Pat. No. 4,623,364, discloses an abrasive grain comprising essentially alpha alumina polycrystalline of high density which forms no cell consisting of the arms extending in a radial direction from the center of the cell and having essentially crystallographically identical orientation, said alpha alumina having a particle size of below 1 micron and said abrasive grain having a hardness of at least 18 Gpa, which is obtained by adding alumina seed crystals to alpha alumina precursor and sintering at 1400.degree. C. or lower. Also disclosed are ceramic bodies having a part of alpha alumina replaced with MgO or zirconia in the form of spinel.
Japanese Patent Publication No. 2-53475 discloses the process for obtaining abrasive grains superior in grinding works made of stainless steels etc. comprising mixing aqueous dispersion of alpha alumina monohydrate with such amount of aqueous dispersion of yttrium compound that at least 0.5 wt % of yttria is included in a product after sintering, setting and drying the mixture, crushing the dried solid to grits, and calcining the grits to remove substantially volatile substances from the grits.
Japanese Patent Application laid open No. 64-11183 which corresponds to U.S. patent application Ser. No. 54619 which relates to U.S. Pat. No. 4,881,951 discloses abrasive grits, method for producing the same and the products made therewith, comprising alpha alumina and a reaction product of aluminium oxide and at least about 0.5 wt %, preferably about 1-30 wt % of rare earth metals selected from the group consisting of praseodymium, samarium, ytterbium, neodymium, lanthanum, gadolinium, cerium, dysprosium, erbium and mixtures of two or more of such rare earth metals.
The International Publication WO 90/08744 discloses the process for producing alpha alumina ceramic bodies or abrasive grains having a crystal size of below 0.2 micron either in average or substantially, comprising effecting the gelation of an alumina sol containing minute alpha alumina crystals, heating the gel from 900.degree. C. to 1100.degree. C. within 90 sec, and holding it at 1,000 to 1,300.degree. C. for a given time.
In Japanese Patent Application laid open No. 2-283661, highly dispersible alpha alumina hydrate is mixed with dilute acid solution, and cerium compound is added in an amount of about 0.01-2 wt % in a form of CeO.sub.2 relative to the alumina content. The suspension thus obtained is deaerated and deflocculated and subsequently dried and crushed. Sintering is conducted in course of several stages, including sintering under gas pressure. Thus, fine crystalline ceramic materials consisting of fine crystalline matrix and whisker-like needle crystal including 0.01-2 wt % cerium compound can be obtained.
In accordance with Japanese Patent Application laid open under No. 3-234785 which corresponds to U.S. Pat. No. 5,190,567, there have been disclosed sintered alumina grains having the strength and hardness equal to or higher than conventional abrasive grains, and having superior properties in grinding resistance and wear resistance and superior self-sharpening property in course of grinding, which comprise aluminum oxide in an amount higher than 98 wt % and lithium oxide in an amount of 0.01-1.5 wt %, wherein the crystallization ratio of the alpha alumina crystal is 75-95 % and the alpha alumina crystal size is less than one micron, preferably 0.1-0.5 micron, which are obtained by mixing alumina monohydrate with a lithium compound and optionally with aqueous compound of Mg, Ca, Co, Ni, Cr, Fe, Sl, Zn, Mn, Ti or Zr as grain growth inhibitors or for upgrading the toughness of the grains, treating the mixture with ultrasonic vibration to obtain alumina dispersion, drying and crushing the dispersion and sintering the crushed objects at a temperature of 1100.degree.-1500.degree. C. for ten minutes to 2 hours.
There also have been provided methods for producing abrasive grains based on aluminum-rare earth metal oxides as follows.
U.S. Pat. No. 3,802,893 discloses an abrasion-resistant polycrystalline ceramic having a grain size of from about 2 to about 5 microns and a density of at least 3.90 grams/cc, which consists essentially of about 99.5 wt % to about 99.9 wt % of aluminum oxide, about 0.01 wt % to about 0.25 wt % of magnesium oxide and about 0.01 wt % to about 0.25 wt % of samarium oxide, which is produced by a process comprising blending aluminum oxide, magnesium oxide, samarium oxide, screening the dried mixture to obtain free-flowing powder particles, pressing the dried powder to form a green part having a desired shape, presintering the green part to remove the organics and firing the part in a hydrogen atmosphere at a temperature above about 1500.degree. C. for about 5 hours.
Japanese Patent Application laid open No. 59-102865 discloses the process for producing ceramic tools, comprising mixing 0.05-3.0 wt % of one or more substance selected from the group consisting of Tb.sub.4 O.sub.7, Ho.sub.2 O.sub.3, Er.sub.2 O.sub.3 and Gd.sub.2 O.sub.3, pressing the dried mixture to a desired shape, sintering the object in an inactive gas to obtain a theoretical density of 95-99% and subjecting the object to a hot isostatic press to obtain a theoritical density above 99.5%.
The following are references where the sintering performance and the fine structure are modified by adding rare earth metals to alumina.
1. In the Ceramic Association Journal(Yogyo-Kyokai-Shi), 87(12), p.632-p. 641, under the title of "Effects of Rare Earth Oxides on Sintering of Alumina", there is described the effects of addition of rare earth metal oxides (Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Er.sub.2 O.sub.3) on alumina sintering. It is reported that any one of these substances suppress the densification of alumina up to 1500.degree. C., but at 1500.degree. C. Sm.sub.2 O.sub.3 and Er.sub.2 O.sub.3 enhance and Y.sub.2 O.sub.3 and La.sub.2 O.sub.3 inhibit the densification respectively. Above 1700.degree. C., all rare earth oxides promote the densification of alumina, especially, the addition of Er.sub.2 O.sub.3 shows the best promoting effect on the densification. PA1 2. In the Ceramic Association Journal(Yogyo-Kyokai-Shi), 88(9), p.531-p.538, under the title of "Effects of Er.sub.2 O.sub.3 Addition on Sintering of Alumina", there is described the effects of Er.sub.2 O.sub.3 and its content on initial stage sintering of alumina. Observation is made as to the addition of Er.sub.2 O.sub.3 of 0.05-2.00 wt % and it is reported that the addition up to 0.50 wt % accelerates the densification but no remarkable effect is observed above 0.50 wt %. PA1 3. In the Ceramic Association Journal(Yogyo-Kyokai-Shi), 88(11), p.666-p.673, under the title of "Effects of Er.sub.2 O.sub.3 Addition on Final Stage Sintering of Alumina", there is described the effects of addition of Er.sub.2 O.sub.3 of 2.00 wt % to alumina, especially the effects on the sintering performance and microstructure on the final stage sintering of alumina. It is reported that the solid state reaction between Al.sub.2 O.sub.3 and Er.sub.2 O.sub.3 is completed at 1700.degree. C. and 3Er.sub.2 O.sub.3 .multidot.5Al.sub.2 O.sub.3 produced as a secondary ingredient provides an influence such that corundum grains of alumina specimens trap small pores inside of them and the distribution range of grain size tends to extend widely, while specimens containing Er.sub.2 O.sub.3 trap little pores and provides sintered particles having uniform particle size and high density.
However, these abrasive materials consisting of alumina-spinel, alumina or alumina-rare earth oxide are not satisfactorily accepted from the following viewpoint.
In Japanese Patent Publication No. 1-54300, there exists a modifying component such as at least 10 vol % of zirconia and/or hafnia and at least 1 vol % of spinel derived from alumina and at least one oxide of a metal selected from cobalt, nickel,zinc, or magnesium, between alumina particles as main component, so that the hardness of the abrasive grains is as low as below 18GPa while commercial fused alumina abrasive materials have the hardness of 20-22 GPa. The reason for reduction of the hardness is conjectured to be that the hardness of said modifying component is lower than that of corundum, for the hardness of ZrO.sub.2 or HfO.sub.2 is about 10-12 GPa and the hardness of the spinel derived from alumina and at least one oxide of a metal selected from cobalt, nickel,zinc, or magnesium is 14 -18 GPa.
In Japanese Patent Publication No. 4-4103, the hardness is reported to be above 18 GPa (above 20-21 GPa in Embodiments). But, the abrasive grain is composed of a single crystal phase comprising essentially alpha alumina polycrystalline of high density which forms no cell consisting of the arms extending in a radial direction from the center of the cell and having essentially crystallographically identical orientations, so that it is difficult to prevent forming micro-pores on the crystalline grain boundaries and also it is difficult to relax the thermal stress caused in course of grinding. As a result, the grinding property for grinding hardly ground materials such as stainless steels, titanium steels, high nickel alloys, aluminum and so forth is inferior. Besides, when the alpha alumina is partly replaced with magnesium or zirconia of spinel, hardness reduction is unavoidable as well.
In Japanese Patent Publication No. 2-53475 and Japanese Patent Application laid open No. 64-11183, the reaction product, that is aluminum oxide-rare earth metal oxide is produced by mixing aluminum oxide and rare earth metal oxide. The addition of Y, Gd and Dy yields garnets (of cubic structure), while the addition of Pr, Sm, Yb and Er yields perovskites (of orthorhombic structure, garnets may be partly included). Such mineral phases, according to observation by a transmission electron microscope, are included in the ring structure which encircles an alpha alumina crystal domain as well as alpha alumina crystal domain. For example the addition of Y.sub.2 O.sub.3 composes the mineral phase of yttrium-aluminium garnet(garnet phase: 3Y.sub.2 O3.multidot.5Al.sub.2 O.sub.3) which is included in the ring structure encircling the alpha alumina crystal domain and also included in the alpha alumina crystal domain of 1-1.5 microns in diameter. Ceramic abrasive grains of this crystal structure are superior in grinding difficult to grind materials such as stainless steels, titanium steels, high nickel alloys and aluminum. However, said alpha alumina crystal domain is as large as 1-1.5 microns in diameter, so that the self-sharpening property obtained by fine crystals is reduced. As a result the grinding property deteriorates. Furthermore, such structure requires at least about 0.5 wt % (preferably about 1-30 wt %) of rare earth metal oxides which forms a garnet phase and perovskite phase of lower hardness as compared with alpha alumina in the alpha alumina crystal domain as a first phase, so that the reduction of hardness is unavoidable. One of the reasons the crystal particle size becomes large is the high sintering temperature. Higher content of rare earth oxide in the aluminum-rare earth metal oxide is hardly sintered, so that the sintering temperature shall be raised to promote the density of the ceramic particles. Higher sintering temperature is not practical in industrial production. In addition the rare earth oxides are too expensive to use in large quantities for abrasive materials.
In International Publication No. WO 90/08744, alumina sol containing minute alpha alumina crystals is gelatinized and sintering is performed under pre-arranged conditions for obtaining a product having a crystal size of below 0.2 micron in average, density of at least 95% of theoretical density and hardness of above 2000 kg/mm.sup.2. The resultant alpha alumina abrasive grains are of high strength and high toughness and of high grinding property. But, as in the case of Japanese Patent Publication No. 4-4103, said grains consist of single alpha alumina phase only, so that the relaxation of the thermal stress caused in course of grinding is not expected.
Japanese Patent Application laid open No. 2-283661 tried to solve the above defects by adding cerium compound of about 0.01-2 wt % in a form of CeO.sub.2 relative to the alumina content to obtain ceramic materials consisting of fine crystal matrix and whisker-like needle crystals. However, further improvement is expected to produce a more satisfactory result and besides, the production method is troublesome because it includes a complicated sintering process under gas-pressure.
Japanese Patent Application laid open No. 3-234785 solves most part of defects in the conventional technology by adding lithium oxide in an amount of 0.01-1.5 wt % in a final product to aluminium oxide, because the addition of lithium source promotes the nucleation of alpha alumina without the addition of spinel components or alpha alumina seed crystals and besides lowers the transition temperature (from theta alumina to alpha alumina) of the dried gel of alumina dispersion, so that the transition can be easily promoted and sintered alumina abrasive grains having a dense, uniform and fine crystal structure is obtained. However, the abrasive grains are substantially composed of a single crystal phase comprising essentially alpha alumina polycrystalline of high density having the alpha alumina particle size of below 1 micron, and consequently it is difficult to relax the thermal stress caused in course of grinding.
U.S. Pat. No. 3,802,893 and Japanese Patent Application laid open No. 59-102865 suggests that physical properties of the ceramic bodies consisting mainly of aluminum oxide can be improved by adding some rare earth metals or oxides thereof at a given amount (Sm.sub.2 O.sub.3 : about 0.01-0.25 wt %, or Tb.sub.4 O.sub.7, Ho.sub.2 O.sub.3, Er.sub.2 O.sub.3 and Gd.sub.2 O.sub.3 : 0.05-3.0 wt %). However, such prior art relates to ceramics or ceramic tools which are effective for cutting knives, and there is no suggestion of effectiveness in ceramic abrasive grains.
In the aforesaid three Ceramic Association Journals (Yogyo-Kyokai-Shi), the effects of addition of rare earth oxides (Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Sm.sub.2 O.sub.3, Er.sub.2 O.sub.3) on sintering of alumina are studied and the acceleration effect of Sm.sub.2 O.sub.3 and Er.sub.2 O.sub.3 on sintering is observed. Especially in the addition of Er.sub.2 O.sub.3 of 0.05-2.00 wt %, the addition up to 0.50 wt % accelerates the densification. However, such literature are thoroughly academic papers. As seen from the experimental processes described therein, reagents are used as starting materials (alpha alumina: purity 99.99%, average grain size: 0.2 micron; Er.sub.2 O.sub.3 : purity 99.9%, average grain size: 0.5 micron) and after subjecting the object to hydrostatic press sintering is conducted at above 1500.degree. C. using a high temperature furnace and consequently ceramic materials are obtained. There is, however, no suggestion of the effectiveness for producing ceramic abrasive grains. Namely, disclosed therein are ceramic materials of high mechanical strength which are obtained by using high-purity materials, preventing anomalous grains from occurring and inhibiting grain growth. There is no suggestion as to whether the product has a self-sharpening property, which is required for abrasive grains, that is, the property of contributing to grinding and self-wearing to reproduce fresh cutting edges, in addition to the regular properties such as hardness and strength.
As described above, the research on ceramic materials does not suggest the applicability to abrasive grains. Furthermore, the conventional ceramic abrasive grains are not satisfactory in hardness, grain strength (toughness), self-sharpening property and relaxation of thermal stress caused in the course of grinding, so that they are not fully satisfactory to grind the difficultly ground materials such as stainless steels, titanium steels, high nickel alloys, aluminum and so forth.